1. Field of the Invention
The present invention relates to an optical system for indication in the viewfinder of a single-lens reflex camera.
2. Description of the Prior Art
Hitherto there have been known a number of optical systems for indication in the viewfinder of a single-lens reflex camera. One of such conventional optical systems is designed to have an additional pentagonal prism bonded to the pentagonal Dach-prism of the viewfinder. The information beam coming from an indication element provided outside of the viewfinder is at first directed to the additional pentagonal prism in which the beam is twice reflected and thereafter led to an eyepiece of the viewfinder through the pentagonal Dach-prism. This optical system necessitates machining the additional pentagonal prism and incurring a substantial cost therefor. For that reason, it cannot produce any remarkable advantage at any rate. Apart from that fact, this optical system has the drawback that it lacks means that permit correction of any possible incorrectness caused by machining errors, mounting errors and the like.
Another example of the prior art optical system for indication in the viewfinder is shown in FIG. 1.
In order to direct the information beam coming from an indication element 1 to an eyepiece 4 through a pentagonal Dach-prism 3, this optical system further comprises a first reflective member 2a, a second reflective member 2b and a concave lens 2c interposed in the light path between said first and second reflective members. The first reflective member 2a is upwardly spaced from the third reflection surface 3a of the pentagonal Dach-prism 3. The second reflective member 2b is adjacent to said third reflection surface 3a. The concave lens 2c is provided to adjust the length of light path. The information beam is reflected by the first reflective member 2a to impinge upon the second reflective member 2b through the light path length adjusting lens 2c. Then, the second reflective member 2b reflects the information beam to the prism 3 so that it may reach the eyepiece 4 after passing through the pentagonal Dach-prism 3.
According to the conventional optical system as shown in FIG. 1, it is possible to compensate, to some extent, for any possible incorrectness caused by machining errors, mounting errors and the like by adjusting the first and/or second reflective members. However, arrangement of said two reflective members should be made so that the second reflective member never interrupts the information beam incident upon the first reflective member. This arrangement caused the optical path to lengthen and, therefore, makes it necessary to additionally provide a concave lens such as the lens 2c so as to adjust the optical path length. As a result, the body size of the apparatus itself in which the optical system is incorporated, has to be enlarged accordingly.